secularism (n.)

c. 1300, seculer, in reference to clergy, "living in the world, not belonging to a religious order," also generally, "belonging to the state" (as opposed to the Church), from Old French seculer, seculare (Modern French séculier) and directly from Late Latin saecularis "worldly, secular, pertaining to a generation or age," in classical Latin "of or belonging to an age, occurring once in an age," from saeculum "age, span of time, lifetime, generation, breed."

This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan also connects it with "bind" words and lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").

The ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years).  Ecclesiastical writers in Latin used it as those in Greek did aiōn "of this world" (see cosmos). It is the source of French siècle "century." The meaning "of or belonging to an age or a long period," especially occurring once in a century, was in English from 1590s.

From mid-14c. in the general sense of "of or belonging to the world, concerned in earthly more than in spiritual, life;" also of literature, music, etc., "not overtly religious." In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s. Related: Secularly.

1846, "one who theoretically rejects and ignores all forms of religion based on revelation;" see secularism + -ist. More specifically by 1851 as "one who maintains that public education and civil policy should be conducted without the introduction of a religious element."

The religionist is noticeably the slave of inherited associations, and always thinks, more or less, within the fetters of the Bible. The Secularist, standing on independent grounds, determines for himself what should be the attitude of man towards the Infinite Personality on the part of those discerning His existence. [George Jacob Holyoake, "Secularism Distinguished from Unitarianism," 1855]

Earlier as "one of the secular clergy" (1716). Related: Secularistic.